The operation of a polymer transistor is described with reference to FIG. 1. The transistor, designated generally by numeral 10, has opposing conductors, namely source 12 and drain 14, separated by a junction region 16 of conducting polymer.
Conducting polymers feature a conjugated carbon backbone. Some common conducting polymers are polyaniline, polypyrrole and polyacetylene. These materials are semi-conductors. However, upon oxidation or reduction of the polymer, conductivity is changed. The oxidation or reduction leads to a charge imbalance which, in turn, results in a flow of ions into the material in order to balance the charge. These ions or dopants enter the polymer from a surrounding, ionically conductive medium, such as a gel, a solid electrolyte or a liquid electrolyte. If ions are already present in the polymer, they may exit when it is oxidized or reduced.
The electrical resistance of junction 16 is a function of the oxidation state of the conducting polymer. Polymer 16 is immersed in an electrochemical solution 18, or gel electrolyte, containing mobile ions, and the oxidation state of junction 16 is varied by changing the potential applied to a gate electrode 19 in solution 18 to drive ions into or out of the junction 16. The state of higher conductivity (or, equivalently, lower resistivity) of the junction 16 is referred to as the `ON` state, while the state of lower conductivity (or higher resistivity) is referred to as the `OFF` state. Of course, description in terms of binary states is merely a descriptive convenience and more states of intermediate conductivity may be defined or employed.
Relevant characteristics of a polymer transistor include the junction resistance in both the low- and high-resistance states, switching speed of the transistor and the amount of charge needed to switch the resistance of the device. It is desirable to provide a method for manufacturing a polymer transistor which allows a small junction size to be fabricated controllably and reliably, so as to decrease resistance in the ON state (thereby minimizing energy loss in the transistor), increase switching rates, and decrease the amount of charge needed for switching. Current techniques provide junctions having widths no smaller than on the order of 50 nanometers. Polymer transistors are currently fabricated using lithographic processes such as described by E. Paul, et al., "Resistance of Polyaniline Films as a Function of Electrochemical Potential and the Fabrication of Polyaniline-Based Microelectronic Devices," 89 J. Phys. Chem., 1441-47 (1985), which is incorporated herein by reference. C. Kranz, et al. in "Lateral Deposition of Polypyrrole Lines over Insulating Gaps," Advanced Materials v.7, n.6, 568-71 (1995) describes employing a scanning microscopy tip to electrochemically deposit lines of polypyrrole on a two-dimensional surface and creating a transistor by connecting the lines. Junction or gap dimensions are not controllable with this method.